A liquid crystal display (LCD) device that has been used typically employs a polarizing film using linearly polarized light. The polarizing film is fabricated by adsorbing an iodine or dichroic dye on a polyvinyl alcohol (PVA) film to form a resulting adsorbed film and stretching the resulting adsorbed film in a certain direction.
The polarizing film fabricated thus cannot be used as a practical polarizing film since its mechanical strength is weak with respect to the direction of a transmission axis and a polarizing function is significantly degraded due to shrinkage by heat or moisture.
In order to solve the above problems, a polarizing film having a structure in which an adhesive is adhered between supports such as an acetic acid cellulose film, etc. has been developed.
The polarizing film using the above-described polyvinyl alcohol film produces linearly polarized light by absorbing light that progresses in one direction while passing only light that vibrates in the other direction. Thus, efficiency of the polarizing film does not exceed 50% theoretically, which acts as the most significant factor to reduce the efficiency and brightness of an LCD.
If a reflective polarizing film formed by using cholesteric liquid crystal is additionally used, the shortcomings of the conventional polarizing films can be improved greatly. Cholesteric liquid crystals have a selective reflection characteristic in which a twisted direction of a spiral liquid crystal structure and an oriented direction of a circularly polarized light are consistent with each other and only a circularly polarized light of which wavelength is the same as the spiral pitch of the liquid crystal is reflected. If the selective reflection characteristic is employed, it is possible to fabricate a polarizing film that converts non-polarized light of a constant wavelength band into a specific circularly polarized light.
That is, if non-polarized light in which a left circularly polarized light component and a right circularly polarized light component are mixed equally is incident to a cholestric liquid crystal film having a left-wind or right-wind spiral structure, a circularly polarized light of which direction is the same as the spiral direction is reflected and a circularly polarized light of which direction is in the opposite direction to the spiral direction is transmitted. At this time, the transmitted circularly polarized light is converted to a linearly polarized light after passing through a ¼λ retardation film. The circularly polarized light reflected thus has its polarization direction changed and transmitted if it is reflected again from a reflective plate. Thus, the whole circularly polarized light can transmit the liquid crystal film. Accordingly, if the polarizing film formed by using the cholesteric liquid crystal film is employed additionally, brightness can be improved significantly compared to the case in which only a conventional absorption type polarizing film that absorbs 50% light is used because there is no loss of light theoretically.
However, a backlight used in an LCD usually generates light of a visible band (400 to 700 nm) that is a region where colors are displayed. Therefore, the selective reflection wavelength region of the cholestric liquid crystal film has to cover the visible light region. If the selective reflection wavelength region of the cholestric liquid crystal film does not cover the visible light region, light of a certain wavelength which is not selectively reflected get into the polarizing film in a non-polarizing state. Accordingly, there is a problem that the picture quality of the LCD is degraded.